Programmable Injectable Dual-Responsive Hydrogels with NIR Ratiometric Self-Monitoring of pH/Ca2+ for Sensing and Actuation

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Abstract
Self-monitoring, functional, injectable hydrogels represent an advancement in smart biomaterials. In this study, we developed injectable dual-responsive hydrogel composites with near-infrared (NIR) emission, fabricated from a nonradiative energy transfer (NRET) nanoprobes and doubly cross-linked microgel (DX MG) building blocks. These hydrogels were easily prepared and exhibit reversible responsiveness to both pH (4.5-7.4) and Ca2+ (0.005-0.08 M) through phototriggered covalent cross-linking of low-concentration (∼7 wt %) shear-thinning microgels. The resulting gels demonstrate tunable modulus and swelling ratios in response to environmental pH and Ca2+ levels, which could be reversibly monitored via NIR ratiometric photoluminescence (RMPL). They withstood 90% compression with low hysteresis, enabling NIR-monitored cyclic compression and tissue imaging. A bilayer actuator, combining DX MG with green-emitting polyacrylamide gels, achieved simultaneous pH/Ca2+ response and tortuosity monitoring via NIR RMPL. To the best of our knowledge, this is the first report of a self-monitoring hydrogel actuator utilizing NIR ratiometric photoluminescence, offering significant promise for PL bionic-driven soft matter and biomedical applications.

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